Supervisory Control
of Untethered Undersea Systems:
A New Paradigm Verified

Presented at the Ninth International Symposium on Unmanned Untethered
Submersible Technology, Durham NH, September 25, 1995.

Abstract

The primary advantage of untethered underwater systems is freedom from cables.
Tethered systems benefit from realtime human control based upon immediate
information received from the remote vehicle. This paper describes a
well-proven system which has the advantages of both, and proposes that the
concepts utilized therein offer exciting new possibilities for exploring and
exploiting the deep ocean.

Introduction

The Advanced Unmanned Search System (AUSS) features an underwater vehicle
which is both unmanned and untethered, yet not strictly autonomous.
Communication with a surface ship is accomplished by means of underwater
sound, as employed by a sophisticated digital acoustic link. Operation is
analogous to radio-controlled robotic space probes: the vehicle generally
proceeds on its own intelligence while transmitting status information and
mission data, but it can receive new instructions at any time. AUSS is, in
fact, far more versatile than a probe, with such abilities as going to a newly
commanded location, hovering at a specified altitude and location, executing a
complete search pattern, or returning home on command.

Acoustic Communication

A popular science and technology magazine, in a recent issue devoted to the
oceans, divided undersea vehicles into three categories: manned submersibles,
remotely operated vehicles (ROVs), and autonomous underwater vehicles (AUVs).
It described ROVs as having unwieldy tethers, "wire, fiber-optic, or
acoustic." The Advanced Unmanned Search System (AUSS) was uniquely depicted
as acoustically tethered. (Presumably, the Hubble Space Telescope is
"electromagnetically tethered.") On the other hand, the Eastern University's
experimental autonomous vehicle (EAVE III) is classified as an AUV with
acoustic telemetry, and Dick Blidburg is referenced as describing
communications as being at the forefront of AUV technology.

At what point does communication make an AUV an ROV? Obviously the practice of
classifying unmanned underwater vehicles as either ROVs or AUVs is
unsatisfactory for free-swimming vehicles with communications, especially
those capable of accepting real- time commands. We will resist the temptation
to introduce yet another acronym (eg., Semi-autonomous Unmanned Untethered
Undersea Vehicles), and will refer to them as semi-autonomous or supervised
vehicles.

Supervisory Control

Just as acoustic telemetry is the only viable means of real-time, long-range
untethered underwater communication, the only form of control is supervisory.
True joystick-type remote control would require full-time downlink
transmission, which would drown out uplink communications, leaving a pilot
both blind and without instruments.

Like a purely autonomous vehicle, a supervisory controlled one is not
encumbered by a cable, the proverbial tail which wags the dog of deep towed or
tethered systems. Thus it can move at relatively high speeds and perform
sharp, precise maneuvers, or it can hover stably without expending power
fighting cable pull. Furthermore, the operators are relieved of tedious
piloting and navigation functions, because all critical loops are closed on
the vehicle. In effect, they have a remote autopilot: they can even direct the
vehicle to go to a location miles away, then go to lunch trusting it to be
waiting there when they return.

On the other hand, with supervisory control, operational decisions need not be
preprogrammed or based on limited machine intelligence. As with a tethered
vehicle, the operator can make operational decisions based on detailed,
up-to-date information, and can immediately see the results of new commands.
Decision thresholds and control loop parameters themselves can be reprogrammed
on the fly. Objects or geographic areas of major interest can be both
identified and fully explored on the same dive rather than on subsequent ones,
thereby saving considerable time and resources. Even the response to critical
or emergency situations can be flexible. If new information warrants, the
operators can make decisions which instantly and totally redirect the mission.

Deep Search

The premise of search is that a known object is lost on the ocean floor, the
location known at best approximately. The objective is to determine the
location of that object, or to verify that it is not within the suspected
area. The two types of general purpose underwater search sensors
employed are sonar and optical. The product of either is images. The width of
sonar images can be hundreds or even thousands of feet, whereas optical images
are usually limited to a few tens of feet. For a given vehicle speed, a sonar
search is much faster than its optical counterpart.

On the other hand, long range is obtained at the cost of resolution. Unless
the object of the search is a nearly intact ship or submarine, sonar is
unlikely to unambiguously distinguish it from worthless objects, termed false
targets. A search system must do more than identify the location of all
likely or possible targets; it must confirm whether each is the object sought.

Generally, a search is not considered finished until a human sees optical
images of the real target or of all potential targets.

AUSS Search And Inspection

The purpose of AUSS is to improve the Navy's capability to locate, identify
and inspect objects on the bottom of the ocean at depths to 20,000 feet. The
vehicle utilizes sophisticated search sensors, computers, and software, and it
is self-navigating. When commanded to do so, it can autonomously execute a
predefined search pattern at high speed, while continuously transmitting
compressed side-looking sonar images to the surface. The operators evaluate
the images and supervise the operation. If they wish to investigate a sonar
contact, they can order the vehicle to temporarily suspend sonar search and
swim over for a closer look using its scanning sonar or still-frame electronic
camera. Each camera image is also compressed and transmitted to the surface.
If the operators see that the contact is not the object sought, a single
command causes the vehicle to resume the search from where it left off.

Exploration of a Skyraider Aircraft

Once the object sought is recognized, a detailed optical inspection can be
conducted immediately. Many options are available during the inspection phase.
Previously transmitted images can be retransmitted at higher resolution. New
optical images can be requested from different altitudes and positions. A
documentary film camera can be turned on or off. AUSS can back away and
perform more sonar scans at higher resolution or longer range. If the object
of interest is very large or found to be highly fragmented, the vehicle can
perform a small photomosaic search pattern, taking overlapping pictures
guaranteeing total optical coverage of a defined area.

Autonomous Search

Imagine an AUV designed to perform deep search. The broad-area, side-looking
sonar phase would be simple, but inspection would be a different matter. A
rather simple AUV could merely swim a predefined search pattern, at a safe
altitude, while recording sonar data. When done, it could be recovered and
dump its data. Humans would study the sonar images, then redeploy the AUV to
fly low over all potential targets and take pictures or photomosaics, when
appropriate. This type of search would be inefficient and costly, requiring
two full deployments and the inspection of all possible targets even if the
very first target was the one sought.

An AUV only slightly smarter than the AUSS vehicle might recognize strong or
unusual sonar returns, and close and photograph each one it finds in the
defined search area, all in one deployment. A really smart AUV might even be
capable of optically recognizing certain objects, and be authorized to
terminate the search when it thinks the job is done. We would want such an AUV
to be conservative, erring to search too much area rather than failing to find
the object or failing to prove it not to be in the search area. It would
therefore investigate many false sonar targets, often "seeing" the sought
object but continuing the search because it wasn't "sure" of what it saw.

In either case, simple or smart, only after the AUV returned and humans
reviewed the optical images could the search director be told whether the
object was found or the area was thoroughly searched. Detailed inspection, if
required, would await an ROV or a manned submersible.

Even if autonomous search were to be proven reliable, it would almost
certainly be slower and more expensive than supervised search. An AUV,
deprived of human expertise, insight and curiosity, could not perform
inspection and exploration as AUSS does.

Generalizing The Concept

We have found supervisory control ideal for deep search and inspection. What
other tasks might be better performed with supervisory control? More simply,
what AUV would not benefit from the ability to communicate?

There are some missions which require acoustic silence, and there are ones
wherein noise or geometry make acoustic communications physically impossible.
There are cases where the mission is so simple or the cost of failure so low
that communication is of little importance. However, if communication can be
used, why not let humans monitor the remote system, handle unexpected
situations, recognize difficult objects and make important decisions in real
time?

A vehicle system designed from the start with supervisory control in mind will
not likely turn out to be just an AUV with an acoustic link added on. It would
probably employ completely different methods than previously considered, which
might result in broader capabilities with a system which is simpler overall
than a purely autonomous one.

Summary

The greatest strength of AUSS is its versatility, the system's ability to
adapt to new situations or knowledge, both during operations and between
operations. Years of operational experience with the system have led to the
evolution of radically different, and highly efficient search tactics. The
attributes of AUSS which have made it a versatile search and inspection system
are primarily a result of acoustic supervisory control. This technology is now
well proven and ready to be applied to a wide range of undersea tasks which
are now performed by towed, tethered, or manned systems.

This technology is covered by U.S. Patents #5,018,114, 4,905,211, 4,432,079 and
4,418,404 and others assigned to the U.S. Government. Parties interested in licensing
this technology may direct inquiries to Legal Counsel for Patents, Code 0012.